34 CHAPTER IV. 



Chromic acid is not a very penetrating reagent, and for 

 this reason, as well as for others, is now seldom used pure 

 for fixing. 



For prolonged hardening it is generally employed in 

 strengths of per cent, to 4 per cent., the immersion lasting 

 a few days or a few weeks, according to the size and nature 

 of the object. Mucous membrane, for instance, will harden 

 satisfactorily in a few days ; brain will require some six 

 weeks. 



Large quantities of the solution must be taken (at least 

 200 grammes for a piece of tissue of 1 centimetre cube 

 Ranvier) . 



In order to obtain the best results you should not employ 

 portions of tissue of more than an inch cube. For a human 

 spinal cord you should take two litres of solution, and change 

 it for fresh after a few days. Six weeks or two months are 

 necessary to complete the hardening. 



I think it is frequently useful to add a little ylycerin 

 there is less brittleness. 



The solution should be taken weak at first, and the strength 

 increased after a time. The objects should be removed from 

 the solution as soon as they have acquired the desired con- 

 sistency, as if left too long they will become brittle. They 

 may be preserved till wanted in alcohol (95 per cent.). It is 

 well to wash them out in water for twenty-four or forty-eight 

 hours before putting the,m into the alcohol. After a timo 

 they generally become green in the alcohol. They may be 

 bleached if desired. 



Chromic acid is a most powerful and rapid hardening 

 agent. (By it you may obtain in a few days a degree of 

 hardening that you would hardly obtain in as many weeks 

 with bichromate, for instance.) It has the defect of a groat 

 tendency to cause brittleness. 



38. Chromic Acid and Alcohol (URBAN PRITCHARD, Quart. 

 Journ. Mic. Sci., 1873, p. 427). Chromic acid, 1 part ; water, 20 parts ; 

 rectified spirit, 180 parts. For hardening such tissues as retina, 

 cochlea, etc. 



A mixture of 2 parts of i per cent, chromic acid solution with 1 part 

 of methylated spirit was once much used by KLEIN (Quart. Journ. Mic. 

 Sci., 1878, p. 315). 



"Roth tin-si' mixtures are irrational (see 37). A still more irratioiuil 



